What was found
DURING CLIMB-OUT FROM MDW, CABIN PRESSURE WARNING AND MASTER WARNING LIGHTS ILLUMINATED. PERFORMED MANUAL OPERATION BUT CONTROL WHEEL WAS HARD TO MOVE. WHEEL MOVEMENT EVENTUALLY LOOSENED AND NORMAL CABIN PRESSURE WAS ACHIEVED. FLIGHT CONTINUED TO DESTINATION (DTW) AND LANDED WITHOUT INCIDENT. FOUND ICE ACCUMULATION ON OUTFLOW VALVE. REMOVED ICE DEBRIS. OPERATIONAL CHECK OK. AIRCRAFT RETURNED TO SERVICE.